Sound Localization Abilities of Florida Manatees, Trichechus manatus latirostris

1. Hugh Buffett Sound Localization Abilities of Florida Manatees,Trichechus manatus latirostris Debborah E. Colbert1, 2, 5, Dr. David Mann1, 2, Joseph C. Gaspard III1, 4, Dr. Gordon B. Bauer1, 3, Kim Dziuk1, Adrienne Cardwell1, & Dr. Roger Reep4 1Sensory Biology and Behavior Program, Mote Marine Laboratory & Aquarium 2University of South Florida 3New College of Florida 4University of Florida 5Sea Life Park by Dolphin Discovery Introduction The Florida manatee lives in an environment where recreational boats are found in high numbers and conspecifics are often out of visual range. How then do they avoid boat collisions and find conspecifics? The manatee auditory system may play a crucial role in accomplishing these challenging tasks.Typical recreational boat engines produce broad-band frequencies that range between 0.01–2 kHz, although they can reach as high as 20 kHz. Manatee vocalizations have fundamental frequencies that range from 2.5–5.9 kHz but can extend to 15 kHz (Nowacek et al., 2003 & 2004). Previous auditory studies have found that manatees can hear over 40 kHz andmay be able to localize higher frequencies (Gerstein, 1999).More recent studies have indicated that manatees have rapid auditory temporal processing (Mann et al., 2005).To further investigate the localization abilities of manatees, two captive-born manatees, Hugh and Buffett, were tested on both a four-choice and an eight-choice sound localization task using different types of sound stimuli in a controlled environment. Methods The subjects were two captive-born male Florida manatees that reside at Mote Marine Laboratory and Aquarium in Sarasota, Florida. At the inception of this study Hugh was 20 years of age and Buffett was 17 years of age. Both had an extensive training history over the previous seven years and were subjects in an auditory evoked potential study. Each subject was trained to position perpendicular to a stationing bar. When a test stimulus was playedfrom one of the underwater speakers, the subject swam to and depressed a speaker. A correct response, touch of the speaker that emitted the sound, was followed by an acoustic secondary reinforcer and food. Subjects were called to station following incorrect responses. Sound location was determined quasi-randomly. Minimum ITI was 30 seconds. All test trials were video-recorded from an overhead camera. Four Choice Experiment Eight Choice Experiment Design & Conditions: Testing was conducted in the center of the Shelf Area with the subject positioned at mid-water depth (0.75 m). Four test speakers were positioned to the front 180 degrees at 45o, 90o, 270o, & 315o at a distance of 105 cm (Figure 1). Three broad-band stimuli (0.2-20, 6-20, & 0.2–2 kHz) were tested at four durations (3,000, 1,000, 500, & 200 ms) and two tonal stimuli (4 & 16 kHz) were tested at 3,000 ms. All stimuli were tested at 100 dB re:1 uPa (±1.5dB). Design & Conditions: Testing was conducted in the deeper Exhibit Area with the subject positioned at mid-water depth (1.37 m). Eight test speakers were positioned at 0o, 45o, 90o, 135o, 180o, 225o, 270o, & 315o relative to the front of the subject’s head at a distance of 3.05 m (Figure 3). Three broad-band stimuli (0.2-24, 18-24 & 0.2–1.5 kHz) were tested at two durations (3,000 & 200 ms). The 0.2-24 kHz, 3000 ms stimuli were tested at several decibel levels (120, 111, 105, 100 & 80 dB re:1 uPa [±1.5dB]). Figure 1. Testing set-up for 4-choice experiment. Figure 3. Testing set-up for 8-choice experiment. Results: Both subjects performed well above the 25% chance level for all of the broad-band frequency conditions (Table 1). Hugh showed a drop in percentage correct as the broad-band signal durations decreased, but this result was not observed with Buffett. Both animals also performed above chance levels with the pure tone signals, but at a much lower accuracy rate than with the broad-band signals. Results: Both subjects performed well above the 12.5% chance level for all of the frequency, duration, and decibel level conditions (Table 2). Hugh’s accuracy declined more rapidly than Buffett’s with decreases in decibel level. Figure 4. Selection distribution for the 0.2-24 kHz at 200 ms. Correct speaker location notated by yellow circles. Figure 2. Selection distribution by frequency conditions (collapsed across durations for broad-band). Correct location notated by yellow circles. Table 1.Overall accuracy performance per subject by frequency and duration conditions (based on 72 trials per condition). Table 2.Overall accuracy performance per subject by frequency, duration and level conditions (based on 80 trials for the 3000 ms and 120 trials for the 200 ms conditions). • Discussion • Both experiments indicated that manatees are able to locate underwater sounds and suggest that they should be able to use sound cues for localizing boats and conspecifics in their natural habitat. Both subjects were capable of localizing the test stimuli. Front-back confusions were surprisingly few and only Hugh had difficulty localizing the test stimuli when the correct speaker was directly behind him with the 200 ms duration. Errors tended to be to the “nearest neighbor” for broad-band stimuli, but were scattered among the locations with no obvious pattern for the tonal stimuli. References Gerstein, E. (1999). Psychoacoustic Evaluations of the West Indian manatee (Trichechus manatus latirostris). Unpublished Doctoral Dissertation, Florida Atlantic University, Boca Raton, FL. Mann, D., Colbert, D. E., Gaspard, J. C. III, Casper, B., Cook, M. L. H., Reep, R. L., & Bauer, G. B. (2005). Temporal resolution of the Florida manatee (Trichechus manatus latirostris) auditory system. Journal of Comparative Physiology, 191, 903-908. Nowacek, D. P., Casper, B. M., Wells, R. W., Nowacek, S. M., & Mann, D. A. (2003). Intraspecific and geographic variation of West Indian manatee (Trichechus manatus spp.) vocalizations. Journal of the Acoustical Society of America, 114 (1), 66-69. Nowacek, S. M., Wells, R. S., Owen, E. C. G., Speakman,T. R., Flam, R. O., & Nowacek, D. P. (2004). Florida manatees, Trichechus manatus latirostris, respond to approaching vessels. Biological Conservation 119, 517-523. • Special Thanks To: • Jann Warfield • Jay Sprinkle • Joe Lappin • Manatee Care Team Interns • New College Students 0.2 - 20 kHz 6 – 20 kHz Hugh Percent Selected Buffett 0.2 - 2 kHz Percent Selected 4 kHz 16 kHz Shelf Area 90o 45o 180o Deep Exhibit Area 135o 225o Shelf Area 105cm Percent Selected 315o 90o 270o 270o 3.05 m 45o 315o 0o Deep Exhibit Area